This invention relates to systems and methods for producing purified water from ambient water present in subsurface environments. In particular, the invention relates to systems for purifying ambient water in a subsurface environment, wherein the systems incorporate ultrafiltration devices.
Water flooding for enhanced oil recovery (EOR) has been used for more than 75 years and accounts for a substantial portion of current oil production in the United States. Water flooding is used to extract the immobile oil present in the reservoir that would not otherwise naturally flow out of the reservoir. Usually this is a secondary recovery technique, however, it is being considered as well in primary and tertiary production for increased ultimate recovery.
In conventional water flooding processes, injection water may be taken from nearest available sources with little consideration to its chemical make-up. Sources of injection water onshore include rivers and aquifers, while seawater is used offshore. Water is usually filtered to remove particles to avoid clogging of the formation. Certain reservoirs require sulfate removal from the water to be used in the flooding process in order to reduce the formation barium sulfate and strontium sulfate scale within the reservoir. In a typical water flooding EOR protocol water is injected at a pressure approximately 500 psi (35 bars) higher than reservoir pressure. Single phase water injection pumps are typically used in subsea applications.
Low salinity water flooding (LSF) is a process of flooding the oil reservoir with water of known and suitable salinity in order to economically extract additional oil from the sandstone and carbonate reservoirs. Since the pioneering work by Tang and Morrow (See for example Guoqing Tang and Norman R. Morrow, Oil Recovery by Waterflooding and Imbibition—Invading Brine Cation Valence and Salinity, SCA-9911, 1999) the benefits of low salinity flooding have been demonstrated in both laboratory and field studies. LSF has been shown to produce 2% to 12% of additional oil than might otherwise have been produced using conventional flooding techniques.
The use of LSF in an oil field can make other chemical and polymer EOR flooding techniques more efficient and can provide cost savings by reducing chemical consumption while increasing hydrocarbon yields. Interestingly, there appears to exist an optimal range of salinity for a specific oil reservoir. The optimal salinity is believed to depend on reservoir characteristics such as mineralogy, formation water chemistry, oil composition, surface chemistry, formation pressure and temperature. The optimal salinity level is typically in the range of 1,000 to 10,000 ppm total dissolved solids (TDS).
In subsea oil field operations, currently available options for producing low salinity water for use in EOR flooding protocols include (1) installing a water treatment system on a topside platform and piping the product low salinity water to an injection well head on the sea floor, and (2) installing a subsurface water treatment system adjacent to the injection well on the sea floor. The first option is made unattractive by the high cost of piping and the limited space available on topside platforms. The second option, though attractive in that it obviates the need for high cost piping and limited platform space, is made unattractive by the subsurface environment itself which frequently has a high concentration of particulate matter which can severely limit the time interval during which a subsurface water treatment system may be operated without maintenance.
Thus, there is a need for new and more robust systems and methods for producing purified water in subsurface environments.